Previously, it has been common to treat a wide variety of materials and material mixtures using an apparatus operating on the principle of a double-action impact mill which functions, for example, as a very efficient grinder or mixer. The apparatus has been presented earlier in the Finnish patent publications FI 94030 B, FI 105112 B and FI 105699 B, and in the PCT publication WO 96/18454.
Apparatuses operating on the principle of a double-action impact mill can be used for treating a wide variety of solid substances or mixtures containing solid matter. The apparatuses can be used, for example, for grinding and sludging mineral substances and fiber-rich materials, for defragmenting substances containing various biomaterials prior to their biological decomposition, or for mixing sludges, pastes and doughs.
The dwell-time of the material to be treated in an apparatus operating on the principle of a double-action impact mill is very short, often less than 0.1 seconds, which is advantageous in many processes. In some cases, however, the dwell-time may be too short for a desired treatment. In such cases, the treated material must be handled-further in a separate additional treatment stage.
A separate additional treatment may be necessary, for example, in order to improve the desired effect of the chemicals that are added to the material to be treated. This is the case, for example, when the chemical added to the material lacks enough time to become absorbed into the substance in the double-action impact mill.
It is easy to treat materials with a very high dry matter content and/or high viscosity in an apparatus operating on the principle of a double-action impact mill. It may, however, be difficult to discharge the treated material with a very high dry matter content and/or high viscosity from the apparatus, and to transport it to the next process stage. An improved result has been achieved by treating the material in two or several sequential apparatuses operating on the principle of a double-action impact mill, in which case it has been possible to lower the viscosity to a superior level. When considering the entire process it would be advantageous, however, if additional further treatments were not necessary at all, or if they could be substantially shortened.
A solution for the material discharging problem from the grinder, operating on the principle of a double-action impact mill, has been suggested in the Finnish patent FI94030. It is essential for the solution, that the material to be treated does not fill the apparatus completely at any stage, i.e. the material remains discontinuous and is surrounded by air at all times. The material content of the apparatus is kept continuously small. This is accomplished by allowing the rotors to rotate at high speeds, by scraping all the material from the walls of the apparatus, and discharging all the scraped material immediately from the apparatus through a relatively large discharge opening. A tangential discharge pipe has been fitted at the discharge opening, where it is also possible to fit an open helix-screw. The discharge opening of the apparatus is increased in size, even to over 90% of the outermost ring of the apparatus. The purpose is thus to ensure that all the material is easily and completely discharged from the apparatus as soon as it encounters the discharge opening.